1. Field of the Invention
The present invention relates to an improved structure of laminated piezo ceramic transformer device, wherein the internal electrode is circular and surround by a circular solid and or dotted line supporting structure to prevent internal crack, a protection coating is apply to the outside of the device to reduce noise level and prevent oxidation and insulation degradation.
2. Description of the Prior Art
The piezo ceramic transformer was first developed in early 1960s based on U.S. Pat. No. 2,974,296. It has many advantages compared to conventional magnetic transformer: low profile, high efficiency, less EMI, unique dependence of step-up ratio on load impedence etc. These advantages match perfectly with recent mobile communication and LCD display needs, more specifically, as a step-up transformer in inverter circuit for CCFL backlight unit.
Many variation and improvement were developed and disclosed in U.S. Pat. Nos. 5,929,554, 5,962,9546 and 278,226 and Japan Pat. No. 11-004025, 2000-226750 and 2000-353833. The original Rosen type design use rectangular laminating piezoelectric transformer which operates in the length longitudinal-oscillation mode by which the laminating was carried out in the thickness orientation. Recent improvement use near square or square type with internal electrode, which operates by the diameter breadth oscillation mode and has achieved higher electromechanical coupling coefficient.
FIG. 1A, FIG. 1B, FIG. 1C and FIG. 1D is top view of single layer piezo ceramic plate of a conventional laminated piezo ceramic transformer, wherein single layer piezo ceramic plate 1 has mesh-printed internal electrode 10 and four connecting electrodes 11, 12, 13 and 14 connect to four different outer electrodes. Internal electrode material is silver paste or other precious metal alloy paste.
FIG. 1E is cross sectional view of conventional laminated piezo ceramic transformer green body, produced by laminating multiple layers of piezo ceramic plate as shown in FIG. 1A, FIG. 1B, FIG. 1C and FIG. 1D. Since only central circular area of piezo ceramic plate has internal electrode printed while outer rim area was not printed to avoid short-circuit or insulation problem, thickness is higher in central area than outer rim. When laminating multiple layers of piezo ceramic plate and applying pressure to form conventional piezo ceramic transformer green body, plate broken or crack is easily introduced, this problem is getting worse when piezo ceramic plate is thinner, internal electrode is thicker or more laminated layer is required.
In addition, above laminated piezo ceramic transformer green body require numerous post processing work, like debinding, sintering, grinding, outer electrode soldering etc. Above-mentioned thickness difference will introduce much higher stress at the center of piezo ceramic plate during post processing.
In addition, since internal electrode will accelerate ceramic crystallization or crystal growth, crystal growth rate and crystal boundary shrinkage rate between central and outer rim area of piezo ceramic plate is different. All these will add up producing crack 22 or micro crack 23 during post processing and significantly reduce yield rate. FIG. 1F, FIG. 1G and FIG. 1H are bottom, top and side view of conventional laminated piezo ceramic transformer with crack and micro crack after post processing.
Accordingly, the object of the present invention is to provide an improved structure of laminated piezo ceramic transformer, mesh-printed with circular solid and or dotted line supporting structure to effectively improve broken or crack introduced by known thickness difference during laminating and post processing work.
Another object of present invention is to provide an improved structure of laminated piezo ceramic transformer, by maximizing electrode material coverage on piezo ceramic plate to accelerate crystal growth rate with lower sintering temperature and lower production cost.
In order to achieve above objectives, the present invention provide an improved structure comprising multiple thin square piezo ceramic plate, mesh-printing with electrode material in three different patterns, a circular internal electrode, a circular solid and or dotted line supporting structure and a connecting electrode. Thin piezo ceramic plate use easy to produce square shape. Circular internal electrode is mesh printed with silver or alloy paste and polarized to present diameter breadth oscillation mode. Connecting electrode is at the outer rim of circular internal electrode, which connect circular internal electrode to outer electrode soldered to post-processed laminated piezo ceramic transformer. Connecting electrode can point to any one of four directions, i.e. first, second, third and fourth direction perpendicular to the edge of piezo ceramic plate. Circular solid and or dotted line supporting structure is mesh printed with same silver or alloy paste and locate at the outer rim of circular internal electrode, excluding connecting electrode area.
Multiple layers of thin piezo ceramic plate with mesh-printed circular internal electrode, connecting electrode and circular solid and or dotted line supporting structure are laminated using first and second or third and fourth direction sequence to produce the laminated piezo ceramic transformer green body in the implementation example. The green body is post-processed with debinding, sintering and grinding then apply outer electrode at four sides connecting internal electrode to produce a finished laminated piezo ceramic transformer 4.
Since electrode material is easy to oxidize, especially when voltage is applied and in high humidity environment, such electrode oxidization will cause insulation degradation. The present invention applies a soft protection coating to prevent high humidity insulation degradation problem in the implementation example. The protection coating can be bond, glue or paint and the protection coating can apply to various laminated piezoelectric devices as shown in FIG. 3A and FIG. 3B.
The present invention and implementation example use thin piezo ceramic plate mesh-printed with circular internal electrode and circular solid and or dotted line supporting structure to prevent uneven plate thickness problem in conventional laminated piezo ceramic transformer and solved known green body plate broken and crack problem.
Since electrode material will accelerate ceramic crystallization or crystal growth, crystal growth rate and shrinkage rate between electrode covered area and uncvered area will be different, this will create uneven and over stress when debinding, sintering and grinding and produce crack or micro crack.
The present invention and implementation example has maximize electrode coverage area on piezo ceramic plate by using solid and or dotted line supporting structure mesh-printed with same electrode material, it not only solved uneven and over stress problem known to conventional laminated piezo ceramic transformer but also reduce sintering temperature as a result of higher crystal growth rate.
The above and other objects, feature and advantages of the present invention will become clear from the following description based upon the accompanying drawings, which illustrate examples of preferred embodiment of the present invention.